Traffic Volume Studies

Dr Rawid

Traffic Volume

• Traffic volume-- is defined as the number of vehicles (orpersons) that pass a point on a transportation facilityduring a specified time period, which is usually one hour.

• In traffic engineering studies there are many volumessuch as daily volume, hourly volume, peak hour volume.In addition volumes of a day or an hour can vary greatly,depending on the different day of the week or differenttime period of a day.

• Benefits: Traffic volume studies are conducted todetermine the number, movements, andclassifications of roadway vehicles at a given location.

Benefits

• The traffic volume data can identify critical flowtime periods, determine the influence of largevehicles or pedestrians on vehicular traffic flow,or document traffic volume trends.

• The length of the sampling period depends onthe type of count being taken and the intendeduse of the data recorded.

• Example: An intersection count may beconducted during the peak flow period. If so,manual count with 15-minutes intervals could beused to obtain the traffic volume data.

STUDY METHODS

• Two methods are available for conducting trafficvolume counts: (1) manual and(2) automatic.

• Manual counts are typically used to gather datafor determination of vehicle classification, turningmovements, direction of travel, pedestrianmovements, or vehicle occupancy.

• Automatic counts are typically used to gatherdata for determination of vehicle hourly patterns,daily or seasonal variations and growth trends, orannual traffic estimates.

Selection of study method

• The selection of study method is determined using thecount period.

• The count period should be the time of day, day ofmonth, and month of year for the study area.

• Count periods may range from 5 minutes to 1 year.• Typical count periods are 15 minutes or 2 hours for

peak periods, 4 hours for morning and afternoonpeaks, 6 hours for midday, and 12 hours for daytimeperiods.

• For example, if we are conducting a 2-hour peak periodcount, eight 15-minute counts would be required.

Manual Count Method

• Applications of manual counts require small samples of dataat any given location.

• Manual counts are sometimes used when the effort andexpense of automated equipment are not justified.

• Manual counts are necessary when automatic equipment isnot available.

• Manual counts are typically used for periods of less than aday. Normal intervals for a manual count are 5, 10, or 15minutes.

• Traffic counts during a Monday morning rush hour and aFriday evening rush hour may show high volumes and are notnormally used in analysis; therefore, counts are usuallyconducted on a Tuesday, Wednesday, or Thursday.

Manual Count Recording Methods

• Manual counts are recorded using one of three methods:(1) tally sheets,

(2) mechanical counting boards, or

(3) electronic counting boards.

• Tally Sheets: Recording data onto tally sheets is thesimplest means of conducting manual counts.

• The data can be recorded with a tick mark on a pre-prepared field form.

• A watch or stopwatch is necessary to measure thedesired count interval.

Intersection Volume Count Tally Sheet

Mechanical Counting Boards

• Mechanical count boards consist of counters mounted ona board that record each direction of travel.

• Common counts include pedestrian, bicycle, vehicleclassification, and traffic volume counts.

• Typical counters are push button devices with three tofive registers. Each button represents a different type ofvehicle or pedestrian being counted.

• A watch or a stopwatch is also necessary with thismethod to measure the desired count interval.

Mechanical Counting Board

Electronic Counting Boards

• Electronic counting boards are battery-operated, hand-held devices used in collecting traffic count data.

• They are similar to mechanical counting boards, but withsome important differences. Electronic counting boardsare lighter, more compact, and easier to handle. Theyhave an internal clock that automatically separates thedata by time interval.

• Special functions include automatic data reduction andsummary. The data can also be downloaded to acomputer, which saves time.

Electronic Counting Board

Volume count

• For example, there was need to remove four houses andconstruct an apartment complex. This proposed land usechange would affect traffic volume. The city wanted todocument the traffic volumes at the closest intersectionduring the peak flow period of the day. The study wasconducted at a nearby controlled intersection and anuncontrolled intersection. The time period chosen, 7:00a.m. to 9:00 a.m., included the morning peak flow

Example

Example• A city decided to conduct a manual traffic count using

the tally sheet method because they did not haveaccess to a mechanical or electronic counting board.The example tally sheet shows one 15-minute count.There were 71 westbound vehicles. If you multiply thisnumber by eight (eight 15-minute periods in a 2-hourpeak flow), you arrive at 586 vehicles during the peakflow. Typically 2-hour peak flow counts would beconducted once in the morning and once in theafternoon.

• If an apartment complex is introduced, another studymay need to be conducted.

Example conted.

• The apartment complex could increase the trafficvolume. If the traffic volume is increased, there maybe a need for new traffic control. The initial studyprovides a baseline count that can be used in a trafficimpact analysis or a traffic control device evaluation.

Automatic Count Method

• Automatic counts are usually taken in 1-hour intervalsfor each 24-hour period. The counts may extend for aweek, month, or year. When the counts are recordedfor each 24-hour time period, the peak flow period canbe identified.

• Automatic Count Recording Methods: They areportable counters, permanent counters, and videotape.

• Portable Counters: Portable counters serve the samepurpose as manual counts but with automatic countingequipment. Pneumatic road tubes are used to conductthis method of automatic counts.

Pneumatic Road Tube and Recorder

Recorder

Permanent Counters

• Permanent counters are used when long-termcounts are to be conducted.

• The counts could be performed every day fora year or more.

• The data collected may be used to monitorand evaluate traffic volumes and trends over along period of time.

• Permanent counters are not a cost-effectiveoption in most situations.

Videotape/CCTV

• Observers can record count data byvideotaping traffic.

• Traffic volumes can be counted by viewingvideotapes recorded with a camera at acollection site.

• A digital clock in the video image can proveuseful in noting time intervals.

• Videotaping is not a cost-effective option inmost situations.

Steps to an Automatic Count Study

• An automatic count study includes three key steps:• Perform necessary office preparations.• Deploy and calibrate data collection equipment.• Check data and retrieve (protect) equipment.• Perform Necessary Office Preparations During office

preparations, coordinate all data collection activities withappropriate state and local officials, including transportation,traffic, and law enforcement agencies. For example, you maycoordinate with state or local officials in obtaining trafficcontrol for the deployment and recovery of equipment. Thefield team must be briefed on the data collection process toensure that all observers are collecting the same data type.Each piece of equipment should be tested.

Steps conted.

• Deploy and Calibrate Data Collection Equipment:• The portable counter method using pneumatic road tubes

since the other methods are not cost-effective for mostautomatic count study situations. The road tubes areprepared on the roadside to minimize the time each trafficlane is closed. Workers then place the road tubes acrossthe lanes.

• The location of the tubes should be outside the influence ofother factors such as an intersection, major access points,etc. Traffic control should be provided to protect the crew.After placing, the crew should make sure that the tubes arefunctioning properly. Finally, the crew can secure the roadtubes to the pavement. To avoid theft, the recorder mayalso be secured.

EXAMPLE

• Intersection Counts• Intersection counts are used for (1)-timing traffic signals,

(2)-designing channelization, (3)-planning turn prohibitions,(4)-computing capacity, (5)-analyzing high crashintersections, and (6)-evaluating congestion. The manualcount method is usually used to conduct an intersectioncount. A single observer can complete an intersectioncount only in very light traffic conditions.

• The intersection count classification scheme must beunderstood by all observers before the count can begin.Each intersection has 12 possible movements. Theintersection movements are through, left turn, and rightturn. The observer records the intersection movement foreach vehicle that enters the intersection

Intersection Movements

Pedestrian Counts

• Pedestrian count data are used frequently in planningapplications. Pedestrian counts are used to evaluatesidewalk and crosswalk needs, to justify pedestriansignals, and to time traffic signals. Pedestrian countsmay be taken at intersection crosswalks, midblockcrossings, or along sidewalks.

• When pedestrians are tallied, those 12 years or olderare classified as adults. Persons below 12 or of schoolage or younger are classified as children. The observerrecords the direction of each pedestrian crossing theroadway.

Vehicle Classification Counts (VCC)

• Vehicle classification counts are used inestablishing structural and geometric designcriteria, computing expected highway userrevenue, and computing capacity.

• If a high percentage of heavy trucks exists or ifthe vehicle mix at the crash site is suspectedas contributing to the crash problem, thenclassification counts should be conducted.

VCC conted.

• Typically cars, station wagons, pickup andpanel trucks, and motorcycles are classified aspassenger cars.

• Other trucks and buses are classified as trucks.School buses and farm equipment may berecorded separately.

• The observer records the classification of thevehicles and the vehicles’ direction of travel atthe intersection.

Traffic Volume Units

• AADT (Average Annual Daily Traffic) --- average of 24-hours traffic volume at a given location over a full 365-days year

• AAWT (Average Annual Weekday Traffic) --- average 24-hour traffic volume occurring on weekdays over a fullyear, AAWT is computed by dividing the total weekdayvolume for the whole year by 260.

• ADT (Average Daily Traffic) --- average 24-hour volume ata given location for some period of time less than a year.

• AWT (Average Weekday Traffic) --- average 24-hour trafficvolume occurring on weekdays for some period less thanone year.

Traffic Volume

AADT on Seaham Road between 1998 and 2001

6,000

5,000

AADT (axlepair)4,000

3,000

2,000

1,000

01988 1992 1995

Year

Traffic variation

• Daily variation factor (DF): It is defined as ratioof AADT over yearly average volume forparticular day of week (Monday, Tuesday etc.)

• Monthly variation factor (MF) --- is defined asratio of AADT over ADT for particular month ofthe year

Traffic Volume

Traffic VolumeTable Calibration of Monthly Variation Factors

Month

Total Traffic

(vehs)

ADT for Month

(veh/day)

Monthly Variation Factor

(MF)

January 19,840 /31=640 797/640=1.25

February 16,660 /28=595 797/595=1.34

March 21,235 /31=685 797/685=1.16

April 24,300 /30=810 797/810=0.98

May 25,885 /31=835 797/835=0.95

June 26,280 /30=876 797/876=0.91

July 27,652 /31=892 797/892=0.89

August 30,008 /31=968 797/968=0.82

September 28,620 /30=954 797/954=0.84

October 26,350 /31=850 797/850=0.94

November 22,290 /30=743 797/743=1.07

December 21,731 /31=701 797/701=1.14

Total 290,851 AADT=290,851/365=797veh/day

Traffic Volume

• Daily variation factor and monthly variation factorare used to reflect the fluctuation of average dailytraffic over a day or a month.

• If values of DF and MF are greater than 1.0 itindicates that average daily traffic for this day ormonth is lower than AADT.

• If values of DF and MF are less than 1.0 itindicates that average daily traffic for this day ormonth is higher than AADT.

• From engineering design point of view, hourlyvolumes are more often used.

Traffic Volume

• Hourly volumes and peak hour volume: Hourlyvolumes are used to reflect variation(fluctuation) of traffic volume in a day andpeak hour volume is defined as the volume inthe single hour that has the highest hourlyvolume. Peak hour is also called rush hour.

• Rate of flow: It is defined as an equivalenthourly volume for a given interval (intervalcan be 5, 10, 15 minutes)

Rate of traffic flow

Interval Volume Interval Volume

7:00 – 7:05 15 7:30 – 7:35 17

7:05 – 7:10 25 7:35 – 7:40 35

7:10 – 7:15 18 7:40 – 7:45 28

7:15 – 7:20 42 7:45 – 7:50 32

7:20 – 7:25 31 7:50 – 7:55 26

7:25 – 7:30 18 7:55 – 8:00 20

Traffic Volume

• The 15-minute interval is considered the standardtime period used, primarily based on the beliefthat this is the shortest period of time over whichflow rates are “statistically stable”.

• Peak hour factor --- used to reflect the stability ofvolume distribution in an hour. Peak hour factor(PHF) is defined as the ratio of hourly volumedivided by maximum rate of flow.

• PHF = hourly volume/maximum rate of flow

Design Traffic Volumes

Traffic Volume

• If the value of PHF is very small it shows that the comingtraffic flow during peak hour is not evenly distributed. If thevalue of PHF is close to 1.0 it indicates that the comingtraffic flow is evenly distributed.

• The 30th hourly volume: It is defined as an hourly volumeat which its ranking on yearly-counting curve counts 30thamong 8760 hourly volumes in a year. The value of thispoint is considered in the planning and design of roadwayfacilities because the volume-ranking curve begins to“flatten out” after this point.

• The 30th highest hourly volume: It is defined as an hourlyvolume at which its ranking on yearly-counting curvecounts 30th among 365 peak hour volumes in a year. Thisvolume is not often used in practice.

Traffic Volume

• DDHV (Directional design hourly volume): DDHV= AADT x K x D where: K is proportion of dailytraffic occurring during the peak hour; D isproportion of peak-hour traffic traveling in thepeak direction.

• VMT (Vehicle Miles Traveled) or VKT(VehicleKilometers Traveled) --- another parameter toestimate product of volume and average travellength for a trip. This element is often used tomeasure intensity of vehicles.

Traffic Volume

Table Estimation of Vehicle-Miles Traveled on a Limited Network: An Example

Station

8-Hour Count

(vehs)

Link Length

(mi)

Link VMT

(veh-miles)

A 6,967 0.25 1,741.75

1 5,159 0.25 1,289.75

2 5,254 0.25 1,313.50

3 5,820 0.25 1,455.00

4 4,058 0.25 1,014.50

5 8,710 0.25 2,177.50

6 7,274 0.25 1,818.50

Network Total 10,810.50